1. Field of the Invention
The present invention relates generally to gas lasers and, more particularly, to discharge tubes for gaseous ion lasers having an internal gas return path.
2. Background Art
A variety of gas lasers are available for many different commercial applications, such as welding, cutting, and surgery. Each gas laser may have a uniquely structured gas discharge tube in which the laser action is performed. Generally, in the design of the gas discharge tubes trade-offs are made so as to, for example, solve particular problems and/or provide specific advantages, as may be required for particular applications of the laser.
The lower efficiency of ion lasers and level of input energy required to achieve an ionization giving rise to laser action results in generation of a substantial amount of heat which must be dissipated away from the discharge tube. Corresponding with the means chosen to dissipate accumulated heat build-up, ion lasers may be classified as radiatively cooled or conductively cooled. Conductively cooled structures store less heat enabling the tube to operate with lower temperature exterior surfaces which proves advantageous in certain applications.
Whether radiatively or conductively cooled, ion lasers are subject to movement of gas within the tube, a phenomenon well known in the art as gas pumping. Gas pumping may lead to a pressure inequilibrium along the length of the tube which is deleterious to the operation of the discharge tube as a laser. The effects of gas pumping may be overcome through provision of a gas return path through which gas may return from high pressure regions to low pressure regions. The return path may be outside the discharge tube as taught, for example, in U.S. Pat. No. 3,582,821 or inside the discharge tube as taught, for example, in U.S. Pat. No. 3,531,734. Internal returns are advantageous to certain applications in that they are less susceptible to breakage than external returns which must be handled with some care.
It is well known in the art that the gaseous discharge along the axis of the tube must be isolated from the gas flow feedback region sufficiently to prevent gaseous discharge, as caused by migrating ions, from arcing through the gas return path. This has been effectuated in radiatively cooled lasers, for example, in French Pat. No. 2,011,043 through the use of blocking baffles (18) and in several conductively cooled embodiments.
U.S. Pat. No. 3,531,734 and later U.S. Pat. No. 4,378,600 disclose internal feedback conduction-cooled segmented bore ion lasers through use of a large compartmentalized volume of gas in a return region from which the gaseous discharge is isolated, respectively speaking, by metallic cylinders the inner diameter of which serve as bores (element 18 in U.S. Pat. No. 3,531,734) and coaxial metallic cylinders radially removed from thinner bore elements (element 56 in U.S. Pat. No. 4,378,600) and affixed, as by brazing, to heat-conducting cups.
Another conduction-cooled segmented bore ion laser with an internal gas return, U.S. Pat. No. 3,501,714, discloses an "S"-shaped bend (FIGS. 4 and 5) in metallic heat-conducting members which contain peripheral grooves and slots to provide a gas return region isolated from the bore. Finally, in a similar vein, U.S. Pat. No. 3,670,262 discloses the isolation of the bore from the feedback and effectuation of an internal gas return in a conductively-cooled beryllium oxide ion laser through the use of off-axis gas return bores (20).
The embodment addressed in the aforementioned patents each create a specific thermal distribution effect within the laser. For example, in the aforementioned patents, conduction-cooled segmented structures with internal feedbacks employ metallic isolation elements upon which the hot gas impinges. Additionally, a temperature gradient is present along the radial heat conduction path from the bore out through the discharge tube and the isolation members interface with the heat conduction path at various points along that gradiant.
The differences in thermal temperature distribution may cause thermal stress between the elements of the structure due to differential thermal expansion as well as alter the gas conductance characteristics which are highly temperature dependent. In particular, the large volume returns of U.S. Pat. Nos. 3,531,734 and 4,378,600, which provides a greater degree of gas storage fail to provide an effective heat transfer to the coolest parts of the tube, principally the inner wall. In addition, each of the above-referenced metallic return configurations which are cylindrical may create non-parallel field lines and give a radial potential which is not desirable. For example, this radial potential may attract ions into a region where they are of no value to the laser operation.